The teachings of the present invention are applied to silicon steel having a cube-on-edge orientation, designated (110) by Miller's Indices Such silicon steels are generally referred to as grain oriented electrical steels. Grain oriented electrical steels are divided into two basic categories: regular grain oriented and high permeability grain oriented. Regular grain oriented electrical steel utilizes manganese and sulfur (and/or selenium) as the principle grain growth inhibitor and generally has a permeability at 796 A/m of less than 1870. High permeability electrical steel relies on aluminum nitrides, boron nitrides or other species known in the art made in addition to or in place of manganese sulphides and/or selenides as grain growth inhibitors and has a permeability greater than 1870. The teachings of the present invention are applicable to regular grain oriented silicon steel.
Conventional processing of regular grain oriented electrical steel comprises the steps of preparing a melt of electrical steel in conventional facilities, refining and casting the electrical steel in the form of ingots or strand cast slabs. The cast electrical steel preferably contains in weight percent less than about 0.1% carbon, about 0.025% to about 0.25% manganese, about 0.01% to 0.035% sulfur and/or selenium, about 2.5% to about 4.0% silicon with an aim silicon content of about 3.15%, less than about 50 ppm nitrogen and less than about 100 ppm total aluminum, the balance being essentially iron. Additions of boron and/or copper can be made, if desired.
If cast into ingots, the steel is hot rolled into slabs or directly rolled from ingots to strip. If continuous cast, the slabs may be pre-rolled in accordance with U.S. Pat. No. 4,718,951. If developed commercially, strip casting would also benefit from the process of the present invention. The slabs are hot rolled at about 2550.degree. F. (1400.degree. C.) to hot band thickness and are subjected to a hot band anneal of about 1850.degree. F. (1010.degree. C.) with a soak of about 30 seconds. The hot band is air cooled to ambient temperature. Thereafter, the material is cold rolled to intermediate gauge and subjected to an intermediate anneal at a temperature of about 1740.degree. F. (950.degree. C.) with a 30 second soak and is cooled as by air cooling to ambient temperature. Following the intermediate anneal, electrical steel is cold rolled to final gauge. The electrical steel at final gauge is subjected to a conventional decarburizing anneal which serves to recrystallize the steel, to reduce the carbon content to a non-aging level and to form a fayalite surface oxide. The decarburizing anneal is generally conducted at a temperature of from about 1525.degree. F. to about 1550.degree. F. (about 830.degree. C. to about 845.degree. C.) in a wet hydrogen bearing atmosphere for a time sufficient to bring the carbon content down to about 0.003% or lower. Thereafter, the electrical steel is coated with an annealing separator such as magnesia and is final annealed at a temperature of about 2200.degree. F. (1200.degree. C.) for twenty-four hours. This final anneal brings about secondary recrystallization. A forsterite or "mill" glass coating is formed by reaction of the fayalite layer with the separator coating.
Representative processes for producing regular grain oriented (cube-on-edge) silicon steel are taught in U.S. Pat. Nos. 4,202,711; 3,764,406; and 3,843,422.
In recent years, to lower the core loss of regular grain oriented products, attention has been turned to increasing the volume resistivity by raising the silicon content to suppress macro-eddy current losses. However, the expected improvement from higher silicon content has generally not been realized. A typical prior art approach has been to increase both silicon and carbon in particular ratios in an attempt to achieve improved magnetic quality. It has been found that raising carbon and silicon together will make the steel more prone to incipient grain boundary melting during the high temperature ingot/slab heating process and more brittle in subsequent processing after hot rolling. Particularly the handling and cold rolling characteristics of the higher silicon and carbon material are degraded. In the process of making regular grain oriented silicon steel, decarburization to 0.003% carbon or less is required to provide nonaging magnetic properties in the finished grain oriented electrical steel. However, higher silicon retards decarburization, making high silicon, high melt carbon materials more difficult to produce.
The present invention is based upon the discovery that in the production of regular grain oriented electrical steel the nature of the intermediate anneal following first stage of cold rolling, and its cooling cycle, have a marked effect on the magnetic quality of the final product. The volume fraction of austenite formed during the anneal, the austenite decomposition product and the carbide precipitate formed during cooling are all of significant importance. A cooling rate after the intermediate anneal which does not allow for austenite decomposition subsequent to the precipitation of fine iron carbide produces lower permeability, less stable secondary grain growth, and/or an enlarged secondary grain size. Added to this, higher silicon will raise the activity of carbon, increasing the carbide precipitation temperature and producing a coarser carbide. As a result, the problems created by improper cooling after the intermediate anneal are aggravated at higher silicon. The teachings of the present invention overcome these problems.
The present invention is directed to the production of regular grain oriented silicon steel starting with a melt chemistry having a silicon content of from about 3% to about 4.5% and a low carbon content of less than 0.07%. The routing of The present invention follows the conventional routing given above with three exceptions. First of all, the hot band anneal can be eliminated. This is particularly true at the lower end of the above given silicon content range. Preferably, however, the routing of the present invention includes such a hot band anneal.
Second, the present invention contemplates a modified intermediate anneal procedure following the first stage of cold rolling. The modified intermediate anneal procedure preferably has a short soak at a lower temperature than the typical prior art intermediate anneal and includes a temperature controlled, two-stage cooling cycle, as will be fully described hereinafter.
The intermediate anneal cooling practice of the present invention provides for austenite decomposition in the first slow stage of cooling prior to precipitation of fine iron carbide in the second rapid stage of cooling. The short soak feature and austenite decomposition are facilitated by the low melt carbon.
Finally, the routing of the present invention preferably includes an ultra-rapid annealing treatment prior to decarburization. The ultra-rapid annealing treatment improves the overall magnetic quality by improving the recrystallization texture. The ultra-rapid annealing treatment is of the type set forth in U.S. Pat. No. 4,898,626, the teachings of which are incorporated herein by reference.
Briefly, U.S. Pat. No. 4,898,626 teaches that the ultra-rapid annealing treatment is performed by heating the electrical steel at a rate in excess of 180.degree. F. (100.degree. C.) per second to a temperature above the recrystallization temperature, nominally 1250.degree. F. (675.degree. C.). The ultra-rapid annealing treatment can be performed at any point in the routing after at least a first stage of cold rolling and before the decarburization anneal preceding the final anneal. A preferred Point in the routing is after the completion of cold rolling and before the decarburization anneal. The ultra-rapid annealing treatment may be accomplished either prior to the decarburization anneal, or may be incorporated into the decarburization anneal as a heat-up portion thereof.